1. Field of the Invention
The present invention relates to a structure for fixation of a deflection yoke and a focus magnet used in a projection cathode ray tube CRT and, more particularly, to the fixation structure of the deflection yoke and the focus magnet capable of improving resolution of the projection CRT by lessening the magnification of a magnetic lens formed by the focus magnet.
2. Description of the Related Art
In the projection CRT, electron beams emitted from an electron gun mounted within the neck portion thereof form a considerably strong image on a screen of the tube. This image is enlarged through an optical system to be projected on a viewing screen.
The projection CRT has the size of normally about 5 to 7 inches and small size as compared with an ordinary CRT, but needs high brightness and resolution.
FIG. 3 illustrates schematically a conventional projection CRT. AS shown in this figure, a focus magnet 30 for focusing electron beams injected from the electron gun is mounted on the periphery of neck portion 50 of a tube. A deflection yoke 20 for deflecting the electron beams is mounted on the periphery of the funnel portion 12 of the tube. The focus magnet 30 and the deflection yoke 20 are fixed on the outside surface of the tube by clamps 34, 24, respectively.
A method of fixing the focus magnet and the deflection yoke by separate clamps 34, 24 shows low precision. In particular, misalignment between the central axes of the electron beams and that of the focus magnet 30 and the deflection yoke 20 can occur. At that time, a beam spot cannot be accurately formed on the screen.
In the meantime, in case that R.sub.d indicates the beam diameter, M indicates the magnetic lens magnification, and X indicates the virtual crossover by the electron gun, EQU R.sub.d =M.times.X.
As the diameter of the electron beam gets smaller, the picture resolution gets better.
In the projection CRT, the focusing characteristic is theoretically the best when the dimensional ratio of a crossover and an image. The is because, in the case of constant distances in the crossover, the lens and the image, to enlarge the magnification ratio of the lens, the lens thickness should be thickened, resulting in an increased spherical aberration.
As shown in FIG. 3, in case that a indicates the distance from the virtual crossover 46 of the electron gun to the center of the magnetic lens 36 formed by the focus magnet 30 and b indicates the distance from the lens center to the screen, the lens magnification M is shown as: M = b/a. Thus, if a = b, M = 1. At that time, the optimum resolution can be obtained.
Thus, it can be seen that, in the projection CRT, the focus magnet 30 is disposed nearer to the screen to obtain a better resolution.
A conventional method for disposing the focus magnet 30 near the screen 10 is to lengthen the overall length of the neck portion 50. However, this method goes against the tendency of the compact projection CRT and also, there is difficulty in the manufacturing process of the tubes, which results in increased production costs.